Types of Enols and Enolates

Enols and enolates vary in their stability. Enols from monocarbonyls are the least stable at equilibrium. However, enols formed in the absence of acid or base are kinetically stable, as they revert to their keto forms very slowly.

Enolate ions of β-dicarbonyl compounds, like β-diketones, β-ketoesters, and β-diesters, are thermodynamically stable because they are doubly resonance-stabilized by the adjacent carbonyls. The enols of such compounds are stabilized by intramolecular hydrogen bonding.

Phenols, which predominantly exist in the aromatic enol form, are the most stable enols.

Two types of regioisomeric enols are possible for unsymmetrical ketones. Carbonyls without an α hydrogen cannot be enolized.

Since α-hydrogen acidity is proportional to enolate stability, esters, acyl cyanides, and tertiary amides form less stable enols.

Primary or secondary amides and carboxylic acids do not form typical enolates, as deprotonation first happens at the acidic proton on nitrogen or oxygen.